The invention relates to a device for detecting the position of a zoom lens in the magnification varying device of a copying machine which has a zoom lens and a numerically controlled motor for moving the zoom lens to copy an original in a plurality of sizes, e.g. actual size, or with the size of the original image enlarged or contracted.
In general, a numerically-controlled motor requires a position detecting device to determine the number of pulses to be applied to the motor. This detecting device must satisfy the following requirements:
(1) Since the actual or life size magnification is generally used, the device must be able to direct the magnification position of the zoom lens system where an original is copied in life size,
(2) When the copying machine is operated again after the power switch thereof has been turned off, or after the electrical power service has been interrupted, the device must be able to discriminate whether the zoom lens is in the enlargement region or in the contraction region, to thereby return the zoom lens to the actual size magnification position,
(3) The device must be manufactured with inexpensive components and be simple in construction, having a simple electric circuit, and
(4) The device must be able to position the zoom lens with high accuracy.